Fast sweep voltage ramp generator and streak camera using same

ABSTRACT

A fast scanning voltage ramp generator including at least one chain of N two-pole avalanche transistors in series and a set of N+1 zener diodes in series between a high voltage power supply and ground, where N is an integer number≧2. A streak camera can utilize this type of generator.

TECHNICAL DOMAIN

The invention relates to a fast scanning voltage ramp generator and astreak camera using such a generator.

STATE OF PRIOR ART

As described in “Improvement in avalanche—transistor sweep circuitry forelectropic streak cameras” by S. W. Thomas, R. L. Griffith and W. R.McDonald (16^(th) International Congress. “High Speed Photography andPhotonics”. Strasbourg, Aug. 27-31, 1984) streak cameras used for thediagnostic of fast photonic phenomena (in visible light or X-rays) arecomposed of a photocathode that converts photons into electrons, anelectron acceleration and deviation system, and an image recoverysystem.

FIG. 1 diagrammatically illustrates such a streak camera that includes:

a photocathode 10,

a horizontal slit 11,

two deflection plates 12,

an anode 13,

an image recovery system 14.

In this type of camera, the basic phenomenon as a function of time isspread on the recovery screen 14. It is transformed into a function of adistance. The use of such a camera requires the use of a scanninggenerator that outputs a positive ramp applied to one of the deviationplates 12 of the electron beam, +V1, and a negative ramp applied to theother deviation plates 12, −V2. The result is a common mode about theaverage potential of these two plates 12.

The quality of the resulting final images is directly related to thequality of the scanning ramps:

linearity,

speed (in volts/nanosecond),

stability.

For the technology used to make this type of scanning ramp generator,the cameras may be classified as a function of the speed of thephenomena to be observed.

Thus, we refer to:

“microsecond camera”: scanning period>1 μs

“nanosecond camera”: scanning period<1 μs

“picosecond camera”: scanning period<1 ns

“femtosecond camera”: scanning period<100 ps.

A voltage of about 400 volts per deflection plate 12 (or per ramp) isnecessary to obtain a spread (distance AB illustrated in FIG. 1) equalto 30 mm.

The following components must be used to obtain such a voltage:

400 Vμs (“microsecond camera”): triode type vacuum tubes MOSFET typetransistors,

400 V/ns (“nanosecond camera”): avalanche type transistors installed ina chain,

400 V/100 ps (“picosecond camera”): avalanche type transistors installedin a chain. But in this case a high voltage is generated (for example 3kVolts in 1 ns), and only a linear part of about 400 volts is used.

The combination of avalanche transistors in the form of a chain is thusdescribed in “Avalanche transistors provides fast power transients”(Electronic engineering, February 1991).

All ramp generators thus obtained according to known art are usuallycomplicated, close to the limits of the technology and furthermore veryfrequently have a high impedance output. This is applicable forgenerators described in “The development of high peak power solid statepulse generators” by A. K. L. Dymok-Bradshaw, J. D. Hares. P. A. Kelletand J. Westlake (Kentech Company, 26.5.1994), which considers the use ofchains of avalanche transistors for an application to high speed opticalcameras.

The purpose of this invention is to overcome the disadvantages ofdevices according to known art by proposing a generator outputting afast positive ramp and/or a fast negative ramp using the principle ofchains of avalanche transistors.

PRESENTATION OF THE INVENTION

This invention relates to a fast scanning voltage ramp generatorincluding at least one chain of N two-pole avalanche transistors inseries and a set of N power supply elements in series, where N is aninteger number ≧2, for example equal to 8, the emitter and the base ofeach transistor being connected to each other and also being connectedto the collector of the next transistor in the chain, an input circuitgenerating a voltage pulse connected to one of the two transistors atthe end of the chain, characterized in hat the set of N+1 power supplyelements in series is formed by a set of N+1 zener diodes in seriesbetween a high voltage power supply and the ground, and in that thecollector and the emitter of each transistor in the chain are connectedfirstly to the cathode and to the anode of a corresponding zener diodethrough at least one resistance, and secondly to the ground through acorresponding capacitor.

Advantageously, the generator includes two chains of two-pole transistorto generate two positive and negative voltage ramps.

Advantageously, the input circuit in the first chain is connected to thefirst transistor in this chain through an inverter transformer and acapacitor so as to transmit a positive pulse received at the input onthe collector of this first transistor. The emitter of the lasttransistor of the first chain is connected to the output of the positiveramp through a capacitor. The input circuit to the second chain isdirectly connected to the emitter of the last transistor in the secondchain to send it a negative pulse received as input. The collector ofthe first transistor in the second chain is connected to the output ofthe negative ramp through a capacitor. A resistance is placed betweenthe high voltage power supply and the zener diodes in series.

Advantageously each resistance used in each chain is composed of threesurface mounted component type resistances.

Advantageously, each capacitor in each of the two chains is of the“patch” type.

Advantageously, the avalanche transistors are of the FMMT 415 type madeby the Zetex Company.

The invention also relates to a streak camera using a ramp generator asdescribed above.

The ramp generator according to the invention using a particulartopology of printed circuit and particular avalanche components enables:

a very small size (<50 cm³),

a single power supply (about +2500 volts),

an amplitude of about 1000 volts on 50 ohms for each output,

a slope of 10 volts/picosecond, namely 400 volts in 40 picoseconds(application to femtosecond cameras),

stability when starting (jitter) of 5 picoseconds.

ZTX 415 or FMMT 415 avalanche type transistors made by the ZETEX Companyas used normally and as indicated by the manufacturer cannot switch inless than 1.5 ns; in the generator according to the invention, they areused to obtain ramps of 0.1 to 0.13 ns.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 diagrammatically illustrates a streak camera.

FIGS. 2 and 3 illustrate the operating principle of the generatoraccording to the invention.

FIG. 4 illustrates an example embodiment of a fast scanning voltage rampgenerator according to the invention.

FIGS. 5 to 9 represent operational curves of the generator according tothe invention illustrated in FIG. 4.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

The ramp generator according to the invention uses avalanche transistorchains. This type of avalanche transistor is a two-pole transistor thathas a collector current-collector voltage characteristic with a negativeslope, when the base-emitter resistance is low. It forms a very fastswitch which closes controlled either by the base or by exceeding thelimiting voltage of the collector. In the latter case, the base and theemitter are connected together. Operation of this transistor is thensimilar to operation of a conventional splitter. The invention only usesthis second case.

FIG. 2 illustrates the installation of such an avalanche transistor T.The base and the emitter of this transistor T are connected together.The collector of this transistor is connected firstly to the groundthrough a capacitor C and a resistance Ra, and secondly to the +terminal of a DC source E through a resistance R. The emitter of thistransistor is connected firstly to the ground through a capacitor C′ anda resistance Rb, and secondly the − terminal of the source E through aresistance R″.

The DC source E polarizes this transistor T through two and R″ to avalue less than its breakdown voltage.

The two capacitors C and C′ are charged to this value E.

Two cases can then arise:

a positive pulse is applied at point A so that the breakdown voltage canbe exceeded. The effect of this is to make the avalanche transistor Tswitch over and to introduce a positive ramp at point B by dischargingthe two capacitors C and C′,

a negative pulse is applied at point B. This also has the effect ofswitching the avalanche transistor T and introducing a negative ramp atpoint A by discharging the two capacitors C and C′.

The amplitude of the signals output by this type of avalanche transistorgenerator depends on the limiting voltage of the collector. If it isrequired to have larger amplitudes, a chain of transistors T′ is used asshown in FIG. 2.

The transistor T in FIG. 1 is then replaced by N transistors T′ inseries, each transistor T′ having its collector connected to the +terminal of a source of a set of voltage sources E′ arranged in seriesthrough a resistance R′. Each transistor T′ is powered at the limit,like transistor T in the circuit shown in FIG. 1.

In the same way as for the operation described for the single transistorT in FIG. 1, a positive pulse at A will produce a large positive ramp atpoint B (N times the ramp of the single transistor T). Similarly, anegative pulse at point B will produce a large negative ramp at point A.In both cases, all transistors T′ switch over at the same time.

If it is required to have a positive ramp and a negative ramp at thesame time, then two chains of avalanche transistors are necessary likethose shown in FIG. 2; the input and output for each ramp being locatedon different sides—at A or at B.

FIG. 4 illustrates the layout of a generator of positive and negativeramps according to the invention, made using two chains, each with eightavalanche transistors.

This generator includes a first chain of N two-pole NPN type avalanchetransistors T1-T8, in this case 8 of them in series, and a second chainof N NPN ype two-pole avalanche transistors T11-T18 in series, and a setof N+1 power supply elements in series, in this case consisting of zenerdiodes D1-D9 in series between a high voltage power supply HV and theground. In each chain, the emitter and the base of each transistor T1-T8and T11-T18 are connected together and are also connected to thecollector of the next transistor; the collectors and emitters of eachtransistor are connected firstly to the cathode and the anode of acorresponding zener diode through at least one resistance R1-R8 orR11-R18, for example through three surface mounted type resistancesplaced on th printed circuit of the generator, and also to the groundthrough a corresponding capacitor C1-C9; C11-C18.

For each chain, an input circuit used to transmit a voltage pulse isconnected to a transistor at the end of the chain.

The first input circuit is formed from a transistor T20, the base ofwhich is connected to the ground through a resistance R21 in parallelwith the resistance R20 in series with a diode D20, the emitter of whichis connected to the ground and for which the collector is connected tothe cathode of the zener diode D9 through the primary of a transformerTR in series with a resistance R0. It receives a positive pulse at E1and is used to apply a positive control pulse on the collector of thefirst transistor T1 of the first chain of transistors T1-T8.

The second input circuit formed in a similar manner from a transistorT21, a diode D21 and a resistance R22 and R23, receives a negative pulseat E2. IT is used to apply a negative control pulse to the emitter ofthe last transistor T18 in the second chain of transistors T11-T18.

The first input circuit in the first chain T1-T8 is thus connected tothe first transistor T1 in this chain through the inverter transformerTR and a capacitor C21 to transmit a positive pulse received at theinput E1 to the collector of this first transistor T1. The emitter ofthe last transistor T8 in the first chain is connected to the positiveramp output S1 through a capacitor C22.

The second input circuit in the second chain T11-T18 is connecteddirectly to the emitter of the last transistor T18 in the second chainto transmit a negative pulse received at input E2 to it. The collectorof the first transistor T11 in the second chain is connected to thenegative ramp output S2 through a capacitor C29.

A resistance R24 is placed between the high voltage power supply HV andzener diodes D1 to D9 in series.

A very advantageous characteristic of the generator according to theinvention is that the voltages polarizing transistors T1 to T8 and T11to T18 in the two chains starting from the single positive HV powersupply are obtained using zener diodes D1 to D9.

Capacitors C1 to C9 and C11 to C18 are advantageously made using patcheson the printed circuit of the generator according to the invention.Their value red ces from C1 to C9 and from C18 to C11. They are designedto compensate for parasite inductances of the two transistor chains.

Advantageously, the configuration of this generator prevents anydisturbance between transistors T11-T18 and transistors T1-T8.

In one example embodiment, the following components are used:

Diodes

D1-D9: 270 V diodes

D20, D21: 1N4148

Resistances

R0-R19: three 22000 Ω resistances (SMC)

R20, R22: 56 Ω

R21, R23: 68 Ω

R24: 15000 Ω, 1 W

Capacitors

C1-C9 and C11-C18: patches on the printed circuit

C20: 470 pf, 300 V

C21, C22, C23: 470 pF, ceramic 2.5 kV

Transistors

T1-T8, T11-T18, T20, T21: FMMT 415, Zetex SOT 23

Transformer

TR: inverter transformer

DC power supply

HV: +2460 V to +2500 V

Inputs

At E1 and at E2: 2 V pulses

Outputs

S1: positive ramp 1 kV/130 ps

S2: negative ramp 1 kV/130 ps

In the printed circuit technology, this type of generator with a 100×90mm² format can be reduced to the size of two credit cards.

The ramp generator according to the invention described above can beused to obtain positive and negative ramps like those shown in FIGS. 5and 6, of the order of 1000 volts with rise times of 130 picoseconds. Inareas with maximum linearity, as shown in FIGS. 8 and 9, a variation ofabout 500 volts is obtained in 50 picoseconds.

FIG. 7 shows the symmetry of the two ramps illustrated in FIGS. 5 and 6,obtained by offsetting each ramp. Thus, in use with a streak camera, itis possible to add DC voltages to make the curves symmetrical with azero median voltage.

Considering the fact that the output is matched on 50 ohms, thegenerator according to the invention does not need to be in the camera,and the connection can be made through 50 ohms lines.

This ramp generator may advantageously be followed by a balancingcircuit according to known art, such as the Balun, which also acts as animpedance transformer. It is thus possible to obtain two ramps (positiveand negative) each at 2 kV on 200 ohms. Furthermore, the symmetryobtained gives better operating reliability.

It is possible to use special entirely passive Gaussian type filters toobtain slower scanning ranges (1 ns, 2 ns, . . . ) on the deviationplates 12 in the streak camera.

This ramp generator may advantageously be followed by a differentiatingcircuit. Thus, two fine positive and negative pulses can be obtained,with a width of about 50 picoseconds at mid-height and an amplitude of500 volts (which is only limited by the number N of transistors). Whenapplied to an antenna, these pulses can be used to obtain a radarsystem.

What is claimed is:
 1. Fast scanning voltage ramp generator comprising:at least one chain of N two-pole avalanche transistors in series and aset of N+1 power supply elements in series, where N is an integernumber≧2, an emitter and base of each transistor being connected to eachother and also being connected to a collector of a next transistor inthe chain; an input circuit generating a voltage pulse connected to oneof two transistors at ends of the chain, wherein the set of N+1 powersupply elements in series is formed by a set of N+1 zener diodes inseries between a high voltage power supply and ground, and wherein thecollector and the emitter of each transistor in the chain are connectedto a cathode and to an anode of a corresponding zener diode through atleast one resistance, and are connected to the ground through acorresponding capacitor.
 2. Generator according to claim 1, wherein saidat least one chain comprises first and second chains of two-poletransistors to generate a positive voltage ramp and a negative voltageramp.
 3. Generator according to claim 2, wherein the input circuit inthe first chain is connected to a first transistor in the first chainthrough an inverter transformer and a capacitor to transmit a positivepulse received at input on the collector of the first transistor. 4.Generator according to claim 3, wherein the emitter of a last transistorof the first chain is connected to an output of the positive rampthrough a capacitor.
 5. Generator according to claim 2, wherein theinput circuit to the second chain is directly connected to an emitter ofa last transistor in the second chain to send the last transistor anegative pulse received as an input.
 6. Generator according to claim 5,wherein a collector of a first transistor in the second chain isconnected to an output of the negative ramp through a capacitor. 7.Generator according to claim 1, wherein a resistance is placed betweenthe high voltage power supply and the zener diodes in series. 8.Generator according to claim 1, made using printed circuit technology,and wherein each resistance comprises three surface mounted componentresistances.
 9. Generator according to claim 8, wherein each capacitoris a patch capacitor.
 10. Generator according to claim 1, wherein N isequal to
 8. 11. Generator according to claim 1, wherein the avalanchetransistors are FMMT 415 type made by Zetex Company.
 12. Streak camerausing a ramp generator according to claim 1.